Freeroll (poker)A freeroll is a situation that arises during play (usually when only two players remain) before the last card has been dealt, in which one player is guaranteed to at least split the pot with his opponent no matter what the final cards are, but where there is some chance he can win the whole pot if certain final cards are dealt. This most commonly occurs in a High-low split game where one player knows that he has a guaranteed low hand made, his opponent cannot make a better low no matter what the last card is, but the player who is low might possibly catch a lucky card that gives him a straight or flush, winning high as well.
Here's an example from Texas hold'em: Angie holds K♣ 10♣, and Burt holds K♥ 10♥. After round three, the board is A♣ Q♠ J♥ 4♣. Both players have an ace-high straight, the current Nut hand, and so they will most likely split the pot. But if the final card happens to be a club, Burt's straight will lose to Angie's flush. There is no other possible final card that will give Burt more than a split; only Angie can improve, so she is freerolling Burt.
If a player knows he has a freeroll, he can raise the pot with impunity, and often a less-skilled opponent with a good hand who does not realize that he is on the wrong end of the freeroll will continue to put in raises with no possible hope of gain.